Conventionally, in order to improve lateral pressure resistance of a power transmission belt such as a V-belt or a V-ribbed belt, short fibers are added as a reinforcing material to a compression rubber layer. For example, PTL 1 discloses a rubber V-belt in which in a belt provided with an adhesive elastic body layer having a tension member embedded therein and retention elastic body layers (compression rubber layers) located on upper and lower sides of the adhesive elastic body layer, the retention elastic body layer contains chloroprene rubber, a reinforcing filler, a metal oxide vulcanizing agent, bismaleimide, and aramid short fibers, and the aramid short fibers are arranged in a width direction of the belt. In this patent literature, elastic modulus in a grain direction (an orientation direction of the short fibers) is increased by the arrangement of the aramid short fibers, thereby maintaining lateral pressure resistance, and durability is improved.
PTL 2 discloses a power transmission V-belt in which rubber hardness of at least one of a tension rubber layer and a compression rubber layer is set to a range of from 90 to 96°, rubber hardness of an adhesion rubber layer is set to a range of from 83 to 89°, and aramid short fibers are arranged in a belt width direction in the tension rubber layer and the compression rubber layer. This patent literature prevents the occurrence of cracks and separation (peeling) of a tension member from each rubber layer in early stage, and improves lateral pressure resistance, thereby improving high load transmission capability.
In recent years, a power transmission belt is required to improve fuel consumption saving properties in order to improve fuel economy performance by reducing transmission loss of a belt, other than the above-described lateral pressure resistance and durability. For example, PTL 2 describes in paragraph [0005] that when rubber hardness of the belt is increased, bending rigidity is increased, and as a result, transmission loss occur in a small pulley diameter. For this reason, an attempt is made to suppress transmission loss by providing cogs on an inner circumferential side or both an inner circumferential side and an outer circumferential side (back face side) of a V-belt to decrease bending rigidity of a belt. A cogged V-belt is generally known as the belt of this kind.
For the improvement of lateral pressure resistance and durability, it is effective means to increase amounts of a reinforcing material such as high modulus short fibers such as aramid fibers or carbon black, thereby increasing rubber hardness, as described in the above-described patent literatures. However, the increase of rubber hardness leads to the increase of bending rigidity of a belt, resulting in the decrease of bending fatigue performance and the increase of transmission loss of the belt in a small pulley diameter, and this leads to the decrease of fuel consumption saving properties. On the other hand, when rubber hardness is decreased in order to improve bending fatigue performance and fuel consumption saving properties, lateral pressure is decreased and a belt is likely to early reach the end of the life thereof. That is, a series of characteristics of lateral pressure resistance and durability is in a trade-off relationship with a series of characteristics of bending fatigue performance and fuel consumption saving properties. Bending fatigue performance and fuel consumption saving properties can be improved by providing cogs on an inner circumferential side or both an inner circumferential side and an outer circumferential side of a V-belt. However, because rubber hardness is increased for the purpose of maintaining lateral pressure resistance and durability, it is the current situation that fuel consumption saving properties are not yet sufficient. For this reason, a preferred rubber composition (particularly, a rubber composition of a compression rubber layer) is desired.
There is a variable speed belt used in a continuously variable transmission, as the V-belt of this kind. In order to change a transmission gear ratio (a speed ratio between a drive pulley and a driven pulley) in the variable speed belt, the belt moves up and down (or back and forth) in a pulley radius direction on the pulley. If this movement is not smoothly conducted, shear force from the pulley strongly acts, and as a result, peeling occurs between rubber layers (an adhesion rubber layer and a compression rubber layer) or between an adhesion rubber layer and a tension member, and fuel consumption saving properties (not fuel consumption saving properties due to bending rigidity, but fuel consumption saving properties based on the decrease of sliding properties) are decreased. To respond to this, an attempt is made to reduce a friction coefficient and improve sliding properties by adding large amounts of a reinforcing material such as short fibers or carbon black to increase rubber hardness, or projecting short fibers from a frictional power transmission surface. However, coexistence of durability performance (lateral pressure resistance and durability) and transmission efficiency (fuel consumption saving properties), which are in trade-off relationship with each other, is not sufficiently established.
PTL 3 discloses a frictional power transmission belt in which a belt body is wound so as to contact with pulleys to transmit power, in which at least a pulley contact portion of the belt body is formed of a rubber composition containing an ethylene-α-olefin elastomer and a powdery or granular polyolefin resin contained therein. This patent literature has an object to improve low sound-generating properties and abrasion resistance by blending a powdery or granular polyolefin resin to short fibers that cannot be blended in a large amount due to uniformity of a composition and material costs. In the examples of this patent literature, a rubber composition containing 75 parts by mass of carbon black and 25 parts by mass of nylon short fibers blended to 100 parts by mass of chloroprene rubber is prepared. However, this composition is described as a comparative example in which sound pressure is high and abrasion loss is large.
However, this patent literature does not describe fuel consumption saving properties, and additionally, even though this belt is applied to a variable speed belt requiring fuel consumption saving properties, fuel consumption saving properties were low and durability was also low.